You can add contacts to your Address Book after sending a new email. Once you have sent an email to a contact not in your Address Book, you will be prompted to add this contact if you choose.
Alternately, you can click the "Contacts" icon at the top of the page, which looks like a Rolodex. This will bring up your Address Book page. Click the "New" button to add a new contact. Click the "Save" button in the upper left corner when you are finished.
How do I add contacts to my Address Book?
